On-press developable negative-working lithographic offset printing plates are known in the prior art. Fox example, U.S. Pat. No. 5,569,573 teaches lithographic printing plates comprising a laser imaging layer containing microencapsulated oleophilic materials in hydrophilic polymer binders. EP 0 770 495 A1 teaches lithographic printing plates comprising near infrared absorption materials, polymer binders and thermoplastic particles capable of coalescing under heat. U.S. Pat. No. 6,983,694 teach on-press developable negative-working offset printing plates coated with near infrared sensitive coating compositions comprising thermoplastic polymer particles, such as polystyrene or poly(acrylonitrile-co-styrene) particles, non-reactive hydrophilic polymer binder and near infrared absorption dyes.
Also, U.S. Pat. No. 6,261,740 teaches negative-working offset printing plates coated with near infrared sensitive coating compositions containing methoxymethacrylamide copolymers, phenolic resins, iodonium salts and near infrared absorption dyes. U.S. Pat. Nos. 6,124,425 and 6,177,182 teach on-press developable negative-working offset printing plates coated with thermally near-infrared absorbing polymers, which undergo cross-linking reactions via cationic polymerization upon exposure to near infrared radiation. The near infrared chromophoric moieties are functionalized to the polymeric backbone via ether and ammonium bonds. U.S. Pat. No. 6,960,422 teaches negative-working offset printing plates, which contain a near infrared sensitive base-coat compositions comprising molecular near infrared dyes, radical generators, radical polymerizable urethane compounds, reactive polymer binders and other additives.
Moreover, U.S. Pat. Nos. 6,969,575 and 7,001,704 teach on-press developable negative-working offset printing plates having an image-forming layer, which comprise near infrared absorbing microcapsules and acid generator compound. U.S. Pat. Nos. 6,582,882, 6,846,614 and 6,899,994 and co-pending US patent application 2005/0123853 teach on-press developable negative-working offset printing plates, which are coated with thermally imageable compositions containing polymer binders, initiator systems and polymerizable components. The described polymer binders are copolymers having non-reactive polyethylene oxide and polypropylene block, or graft copolymers having non-reactive polyethylene oxide side chains co-polymerized with acrylonitrile, styrene and other monomers. The polymerizable components are viscous liquid oligomers containing multiple acrylic functional groups. The initiator system contains near infrared absorption dyes and radical producing compounds, such as triazine and iodonium salts.
All of these coating compositions and printing plates show some disadvantages such as having a tacky surface which causes difficulties for handling and storage, exhibiting phase separation and/or surface crystallization, being difficult to prepare, requiring high laser power to achieve imaging, having poor substrate adhesion and consequently failing to provide sufficient run length on press, not being developable on-press, exhibiting poor scratching resistance, requiring an over-coating layer and/or a special substrate surface treatment and being expensive to manufacture.
There thus remains a need for new materials and new coatings for lithographic plates that would overcome some or all of the drawbacks of the prior art.